An electrically heated hybrid cavity DFB laser and its fabrication method

Abstract

This invention discloses an electrically heated hybrid cavity DFB laser and its fabrication method. The structure includes an epitaxial structure, a negative electrode metal, a positive electrode metal, and a heating electrode metal. A ridge waveguide is provided on the front side of the epitaxial structure. The epitaxial structure includes a phase-shifting region and an active region along the laser cavity length, with the phase-shifting region close to the reflecting end face. The epitaxial structure of the active region includes an active layer and a grating. The epitaxial structure of the phase-shifting region does not include the active layer and grating but includes a passive waveguide layer. The positive electrode metal covers more than 70% of the ridge waveguide in the active region, and the heating electrode metal is clad on the ridge waveguide of the phase-shifting region and spaced apart from the positive electrode metal. By connecting the active region and the passive phase-shifting region to form a hybrid cavity laser, fast and wide-range laser tuning with current control is achieved, enabling fine mode-hopping-free frequency tuning and facilitating stable output power during tuning.

Description

Technical Field

[0001] This invention belongs to the technical field of lasers, specifically relating to an electrically heated hybrid cavity DFB laser and its fabrication method. Background Technology

[0002] LiDAR employing Frequency Modulated Continuous Wave (FMCW) technology offers advantages such as simultaneous distance and velocity measurement and strong resistance to ambient light interference, making it an important development direction for lidar. FMCW radar requires lasers with narrow linewidths (approximately 100kHz), fast mode-hopping-free operation, and wide tuning ranges (approximately 20GHz). Narrow linewidth semiconductor lasers are small in size and highly efficient, making them ideal for application in the field of FMCW radar light sources.

[0003] Conventional narrow-linewidth semiconductor lasers mainly include: distributed feedback lasers (DFB), distributed Bragg reflector lasers (DBR), and external cavity semiconductor lasers (ECL). Narrow-linewidth DFBs have advantages in large-scale mass production and low cost due to their relatively simple and mature epitaxial and manufacturing processes. However, their mode-hopping-free tuning range is generally small, making it difficult to meet the requirements of a wide tuning range.

[0004] Existing heating techniques heat the entire cavity of a DFB laser, altering the period and equivalent refractive index of the DFB grating and providing a wide tuning range. However, these techniques are prone to mode hopping and tuning instability, and also introduce negative effects such as reduced internal quantum efficiency, decreased output power, and increased threshold voltage. These issues limit their application in FMCW laser radar. Summary of the Invention

[0005] This invention addresses the shortcomings of existing technologies by providing an electrically heated hybrid cavity DFB laser and its fabrication method.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] An electrically heated hybrid cavity DFB laser includes an epitaxial structure, a negative electrode metal, a positive electrode metal, and a heating electrode metal. The front side of the epitaxial structure has a ridge waveguide. The epitaxial structure includes a phase-shifting region and an active region along the length of the laser cavity, with one end of the cavity length being a reflecting end face, wherein the phase-shifting region is close to the reflecting end face. The epitaxial structure of the active region includes an active layer and a grating. The epitaxial structure of the phase-shifting region does not include an active layer and a grating, but includes a passive waveguide layer. The positive electrode metal covers more than 70% of the ridge waveguide of the active region, and the heating electrode metal is clad on the ridge waveguide of the phase-shifting region. The heating electrode metal and the positive electrode metal are spaced apart.

[0008] Optionally, the epitaxial structure of the active region includes, from bottom to top, a substrate, a lower cladding layer, an active layer, and an upper cladding layer, with the grating disposed in the upper or lower cladding layer; the epitaxial structure of the phase-shifting region includes, from bottom to top, a substrate, a lower cladding layer, a passive waveguide layer, and an upper cladding layer, wherein the passive waveguide layer is connected to the active layer. The refractive indices of the active layer and the passive waveguide layer are greater than those of the lower and upper cladding layers.

[0009] Optionally, the front side of the epitaxial structure is further covered with a passivation layer, and the passivation layer has an opening in the active region on the ridge waveguide, through which the positive electrode metal contacts the ridge waveguide; the heating electrode metal is separated from the ridge waveguide in the phase shift region by the passivation layer.

[0010] Optionally, a passivation layer spacer is provided between the opening above the active region ridge waveguide and the heating electrode metal above the phase shift region to separate the heating electrode metal and the positive electrode metal; the passivation layer spacer is not less than 3 μm.

[0011] Optionally, the length of the heating electrode metal accounts for more than 70% of the length of the phase shift region, and the resistance of the heating electrode is greater than 50Ω; the heating electrode metal also includes positive and negative leads, which are located on one side of the ridge waveguide.

[0012] Optionally, the epitaxial structure has two grooves formed on its front side, and the ridge waveguide is formed between the two grooves. The heating electrode metal covers the surfaces of the two grooves and the ridge waveguide.

[0013] Optionally, the phase-shift region occupies 2% to 20% of the cavity length of the laser; the difference between the refractive index of the passive waveguide layer and the equivalent refractive index of the active layer is less than 10%, the photoluminescence wavelength of the passive waveguide layer is less than the operating wavelength of the laser, and the material bandgap of the passive waveguide layer is greater than the material bandgap of the active layer.

[0014] Optionally, the passivation layer is a silicon dioxide or silicon nitride thin film with a thickness of 100nm-1000nm; the heating electrode metal is a platinum or titanium metal thin film with a thickness of 100-1000nm and a width of 5-50um.

[0015] A method for fabricating the above-mentioned electrically heated hybrid cavity DFB laser includes the following steps:

[0016] 1) Grow a primary epitaxial layer of a DFB laser on a substrate, including an active layer and a grating forming layer, define the phase shift region and the active region, fabricate the grating pattern by electron beam exposure or full-system exposure, and form the grating by etching the grating forming layer.

[0017] 2) Etch away the grating forming layer and active layer in the phase-shifted region, and perform secondary epitaxial docking to grow a passive waveguide layer;

[0018] 3) Perform three extensions;

[0019] 4) Fabricate the ridge waveguide;

[0020] 5) Deposit metal onto the ridge waveguide in the active region and the corresponding metal wire bonding region to fabricate the positive electrode metal;

[0021] 6) Deposit metal onto the ridge waveguide in the phase-shift region to fabricate heating electrode metal;

[0022] 7) Deposit negative electrode metal.

[0023] Optionally, between steps 4) and 5), a step is also included to deposit a passivation layer on the surface of the structure formed in step 4) and to open an opening in the passivation layer above the ridge waveguide, the opening being located on the ridge waveguide of the active region.

[0024] The beneficial effects of this invention are as follows:

[0025] 1) A hybrid cavity laser is formed by docking the active region and the passive phase-shifting region. The phase-shifting region of the heating electrode metal is located on the reflecting end face. The change in refractive index caused by heating the passive waveguide with electrothermal power is used to provide fine mode-hopping frequency tuning. The resonant frequency of the laser is mainly determined by the period of the DFB grating and the cavity length of the hybrid cavity. This enables fast and wide-range laser tuning with current control, improves the tuning range and tuning rate of the DFB laser, and aims to meet the needs of FMCW radar applications.

[0026] 2) Introducing heating electrode metal only near the reflective end face has little impact on the junction temperature of the active region, and little impact on the threshold and slope efficiency.

[0027] 3) The phase shift region is a passive structure and has no gain effect; the temperature rise of the phase shift region during the phase shift tuning process has little impact on the gain of the entire cavity, which is beneficial to maintaining stable output power during the tuning process. Attached Figure Description

[0028] Figure 1 This is a top view of the electrically heated hybrid cavity DFB laser of Example 1;

[0029] Figure 2 for Figure 1 Cross-sectional view along the a-a' direction;

[0030] Figure 3 for Figure 1 Cross-sectional view along the b-b' direction;

[0031] Figure 4 This is a top view of the electrically heated hybrid cavity DFB laser of Example 2. Detailed Implementation

[0032] The present invention will be further explained below with reference to the accompanying drawings and specific embodiments. The accompanying drawings are merely illustrative to facilitate understanding of the invention, and their specific proportions can be adjusted according to design requirements. The vertical relationships of relative elements and the definitions of front / back in the graphics described herein should be understood by those skilled in the art to refer to the relative positions of the components; therefore, they can all be flipped to present the same component, and all of this should fall within the scope disclosed in this specification.

[0033] Example 1

[0034] refer to Figures 1 to 3 The electrically heated hybrid cavity DFB laser of Embodiment 1 includes an epitaxial structure 1, a passivation layer 2, a positive electrode metal 3, a heating electrode metal 4, and a negative electrode metal 5. The epitaxial structure 1 includes a phase-shifting region A and an active region B along the cavity length direction (indicated by the x-direction in the figure). One end of the DFB laser cavity length direction is a reflecting end face HR, and the phase-shifting region A is close to the reflecting end face HR. The active region B, from bottom to top, includes a substrate 11, a lower cladding layer 12, an active layer 13, an upper cladding layer 14, and a contact layer 15. A grating 16 is provided within the upper cladding layer 14. The phase-shifting region A, from bottom to top, includes a substrate 11, a lower cladding layer 12, a passive waveguide layer 17, and an upper cladding layer 14. The passive waveguide layer 17 is connected to the active layer 13. No grating is provided in the epitaxial structure of the phase-shifting region A. A ridged waveguide 18 is formed on the front side of the epitaxial structure 1 along the cavity length direction (shown laterally in the figure). A passivation layer 2 covers the surface of the epitaxial structure 1, and the passivation layer 2 has an opening on the ridge waveguide of the active region. A positive electrode metal 3 is disposed in the active region B, covering more than 70% of the active region ridge waveguide 18 and extending to the passivation layer 2 on one or both sides of the active region ridge waveguide (corresponding metal wire bonding area). The positive electrode metal 3 contacts the contact layer 15 on the ridge waveguide through the opening. A heating electrode metal 4 is disposed on the phase-shifting region ridge waveguide 18, separated from the ridge waveguide 18 by the passivation layer 2. The heating electrode metal 4 and the positive electrode metal 3 are separated to achieve electrical insulation. The negative electrode metal 5 is located on the back side of the substrate 11. The active region B provides optical gain and basic grating mode selection functions, while the phase-shifting region A provides fine mode-skipping frequency tuning functions. The length of the phase-shifting region A is 1 / 10 of the cavity length.

[0035] A conventional DFB laser epitaxial structure can be applied to the active region B in this embodiment. The grating 16 can be disposed above or below the active layer 13. The grating type includes a uniform grating, a quarter-phase shift grating, a multi-phase shift grating, and a periodic modulation grating, etc. The period of the grating is determined according to the emission wavelength of the laser. For example, the upper cladding 14 of the active region B includes a grating forming layer, an upper waveguide layer, etc., and the grating is fabricated by etching the grating forming layer.

[0036] The material of the passive waveguide layer 17 is selected based on the laser's operating wavelength, the material of the active layer in the active region, and the refractive index. However, the lattice constant of the passive waveguide layer must be the same as that of the substrate material, and the photoluminescence wavelength (PL) of the passive waveguide layer material must be less than the laser's operating wavelength. Furthermore, the difference in refractive index between the passive waveguide layer material and the equivalent refractive index of the active layer 13 must be less than 10%. The refractive indices of the active layer and the passive waveguide layer must be greater than those of the lower and upper cladding layers. The passive waveguide layer material can be a bulk semiconductor material such as InGaAsP, InGaAlAs, AlGaAs, or InGaAs. For example, for a 1550nm laser, InGaAsP or InGaAlAs can be selected. Alternatively, the material can be the same as that of the active layer 13, but the composition can be changed to ensure that the PL wavelength is less than the laser's operating wavelength and the material bandgap is greater than that of the active layer.

[0037] A conventional DFB laser ridge waveguide structure can be applied to this embodiment. For example, two trenches 19 are etched on the front side of the epitaxial structure 1, with the bottom of the two trenches 19 formed at any position between the contact layer 15 and the active layer 13, and a ridge waveguide 18 is formed between the two trenches 19. The width of the ridge waveguide 18 is, for example, about 2-3 μm, and the width of the trenches 19 is about 15 μm. In this embodiment, the heating metal electrode 4 is disposed on the passivation layer 2, extending along the cavity length direction, with a length accounting for more than 70% of the length of the phase shift region A and having positive and negative access regions with bends at both ends; longitudinally (b-b'), it covers the surfaces of the two trenches 19 and the surface of the ridge waveguide 18. The heating electrode metal 4 is a platinum metal thin film with a thickness of about 500 nm, a width of about 25 μm, and a resistance greater than 50 Ω.

[0038] The passivation layer 2 is a silicon dioxide or silicon nitride thin film with a thickness of approximately 300 nm. As an insulator, the passivation layer serves to isolate current. A passivation layer spacer region 21 is provided between the opening above the active region B ridge waveguide (i.e., the positive electrode metal formation region) and the heating electrode metal 4 above the phase shift region A ridge waveguide to separate the heating electrode metal 4 and the positive electrode metal 3. The length of the passivation layer spacer region 21 is not less than 3 μm.

[0039] When the active layer of the DFB is not modulated, the period and equivalent refractive index of the DFB grating remain stable, and the resonant frequency is mainly affected by the cavity length, specifically expressed as: υ*( <n a >*La+ <n p >*Lp)=m*c / 2. Where v is the resonant frequency, <na> 、 <np>Let be the equivalent refractive indices of the DFB active region and the phase-shifted region, respectively; La and Lp be the lengths of the DFB active region and the phase-shifted region, respectively; c be the speed of light in vacuum; and m be the longitudinal mode number. Keeping the temperature and current of the DFB active region constant, then... <n a *La remains stable. When the refractive index of the material in the phase-shift region is slightly changed, mode m remains unchanged, and the resonant frequency is tuned without mode skipping.

[0040] The refractive index of the phase-shift region is affected by temperature, and its refractive index can be controlled by heating, thereby achieving laser tuning. In this embodiment, a thin metal heating element is introduced at the high-reflectivity film end (HR) of the chip, combined with the epitaxial design of a passive waveguide. Electrothermal power is used to heat the passive waveguide, causing a change in refractive index, which in turn causes changes in phase and wavelength, achieving fast and wide-range laser tuning controlled by current. The laser resonant frequency changes with the heating current in the heating region, and the tuning effect is related to the size of the heating resistor and the thermal resistance. Because the heating element is only introduced at the HR end, the impact on the junction temperature of the entire active layer is small, and the impact on the threshold and skew efficiency is also small. This improves the tuning range and tuning rate of the DFB laser to meet the requirements of FMCW radar applications. Since the phase-shift region is a passive structure and has no gain effect, the temperature rise in the phase-shift region during phase-shift tuning has little impact on the overall cavity gain, which is beneficial for maintaining stable output power during tuning.

[0041] The fabrication method of the above-mentioned electrically heated hybrid cavity DFB laser includes the following steps:

[0042] 1) Grow a primary epitaxial layer of a DFB laser on a substrate, including an active layer and a grating forming layer, define the phase shift region and the active region, fabricate the grating pattern by electron beam exposure or full-system exposure, and form the grating by etching the grating forming layer.

[0043] 2) Etch away the grating forming layer and active layer in the phase-shifted region, and perform secondary epitaxial docking to grow a passive waveguide layer;

[0044] 3) Perform three epitaxy steps to form the upper waveguide layer and the contact layer;

[0045] 4) The ridge waveguide is fabricated using an etching trench process; in this embodiment, the contact layer of the phase shift region is removed, but it may not be removed depending on the requirements;

[0046] 5) Deposit passivation layer, and expose the contact layer by opening the passivation layer on the active region ridge waveguide;

[0047] 6) Deposit metal onto the ridge waveguide in the active region and the corresponding metal wire bonding region to create the positive electrode metal, such as a titanium / platinum / gold stack, which has a low resistance;

[0048] 7) Deposit metal onto the ridge waveguide in the phase-shift region to fabricate heating electrode metal, such as titanium or platinum thin films, which have a large resistance to meet heating requirements;

[0049] 8) Deposit negative electrode metal on the back side of the substrate.

[0050] In this embodiment, at the antireflection film end AR opposite to the reflective end face HR, the opening of the passivation layer extends to the edge; in other embodiments, the opening of the passivation layer may also maintain a certain distance from the edge of the AR end, so that the positive electrode metal at the edge does not contact the ridge waveguide, thereby preventing electrical injection and improving catastrophic cavity surface damage (COD) of the laser end face.

[0051] Example 2

[0052] The difference between the electrically heated hybrid cavity DFB laser in Example 2 and Example 1 is that the negative electrode metal 5 and the positive electrode metal 3 are located on the same surface. Conventionally, the positive electrode metal 3 is a p-type metal and the negative electrode metal 5 is an n-type metal. The n-type doped region of the epitaxial layer (such as the substrate 11 or the lower cladding 12) is exposed by etching the ridge waveguide side of the active region on the front side of the epitaxial layer. The negative electrode metal 5 is disposed on the n-type doped region, and its wire bonding position extends to the same height as the positive electrode metal to provide another wiring connection method. See the details below. Figure 4 , .

[0053] The above embodiments are only used to further illustrate an electrically heated hybrid cavity DFB laser and its manufacturing method according to the present invention. However, the present invention is not limited to the embodiments. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the technical solution of the present invention.< / np> < / na>

Claims

1. An electrically heated hybrid cavity DFB laser, characterized in that: The laser comprises an epitaxial structure, a negative electrode metal, a positive electrode metal, and a heating electrode metal. The epitaxial structure has a ridge waveguide on its front side. The epitaxial structure includes a phase-shifting region and an active region along the cavity length of the laser, with one end of the cavity length being a reflecting end face, wherein the phase-shifting region is close to the reflecting end face. The epitaxial structure of the active region includes an active layer and a grating. The epitaxial structure of the phase-shifting region does not include an active layer and a grating, but includes a passive waveguide layer. The positive electrode metal covers more than 70% of the ridge waveguide of the active region, and the heating electrode metal is clad on the ridge waveguide of the phase-shifting region, with the heating electrode metal and the positive electrode metal spaced apart. The heating electrode metal and the ridge waveguide of the phase-shifting region are separated by a passivation layer.

2. The electrically heated hybrid cavity DFB laser according to claim 1, characterized in that: The epitaxial structure of the active region includes, from bottom to top, a substrate, a lower cladding, an active layer, and an upper cladding, with the grating disposed in the upper or lower cladding; the epitaxial structure of the phase-shifting region includes, from bottom to top, a substrate, a lower cladding, a passive waveguide layer, and an upper cladding, wherein the passive waveguide layer is connected to the active layer, and the refractive indices of the active layer and the passive waveguide layer are greater than those of the lower and upper cladding layers.

3. The electrically heated hybrid cavity DFB laser according to claim 1, characterized in that: The epitaxial structure is covered with the passivation layer on its front side. The passivation layer has an opening in the active region on the ridge waveguide, and the positive electrode metal contacts the ridge waveguide through the opening.

4. The electrically heated hybrid cavity DFB laser according to claim 3, characterized in that: A passivation layer spacer is provided between the opening above the active region ridge waveguide and the heating electrode metal above the phase shift region to separate the heating electrode metal and the positive electrode metal; the passivation layer spacer is not less than 3 μm.

5. The electrically heated hybrid cavity DFB laser according to claim 1, characterized in that: The length of the heating electrode metal accounts for more than 70% of the length of the phase shift region, and the resistance of the heating electrode is greater than 50Ω; the heating electrode metal also includes positive and negative leads, which are located on one side of the ridge waveguide.

6. The electrically heated hybrid cavity DFB laser according to claim 1, characterized in that: The epitaxial structure has two grooves formed on its front side, and the ridge waveguide is formed between the two grooves. The heating electrode metal covers the surfaces of the two grooves and the ridge waveguide.

7. The electrically heated hybrid cavity DFB laser according to claim 1, characterized in that: The phase-shift region occupies 2% to 20% of the cavity length of the laser; the difference between the refractive index of the passive waveguide layer and the equivalent refractive index of the active layer is less than 10%, the photoluminescence wavelength of the passive waveguide layer is less than the operating wavelength of the laser, and the material bandgap of the passive waveguide layer is greater than the material bandgap of the active layer.

8. The electrically heated hybrid cavity DFB laser according to claim 3, characterized in that: The passivation layer is a silicon dioxide or silicon nitride thin film with a thickness of 100nm-1000nm; the heating electrode metal is a platinum or titanium metal thin film with a thickness of 100-1000nm and a width of 5-50um.

9. A method for fabricating an electrically heated hybrid cavity DFB laser according to any one of claims 1 to 8, characterized in that, Including the following steps: 1) Grow a primary epitaxial layer of a DFB laser on a substrate, including an active layer and a grating forming layer, define the phase shift region and the active region, fabricate the grating pattern by electron beam exposure or full-system exposure, and form the grating by etching the grating forming layer. 2) Etch away the grating forming layer and active layer in the phase-shifted region, and perform secondary epitaxial docking to grow a passive waveguide layer; 3) Perform three extensions; 4) Fabricate the ridge waveguide; 5) Deposit metal onto the ridge waveguide in the active region and the corresponding metal wire bonding region to fabricate the positive electrode metal; 6) Deposit metal onto the ridge waveguide in the phase-shift region to fabricate heating electrode metal; 7) Deposit negative electrode metal.

10. The manufacturing method according to claim 9, characterized in that: Between steps 4) and 5), there is also a step of depositing a passivation layer on the surface of the structure formed in step 4) and opening the passivation layer on the ridge waveguide, the opening being located on the ridge waveguide of the active region.

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